Spinal cord stimulation (SCS) is a well-accepted clinical method for reducing pain in certain populations of patients. Spinal cord stimulator and other implantable tissue stimulator systems come in two general types: radio-frequency (RF)-controlled and fully implanted. The type commonly referred to as an “RF” system includes an external transmitter inductively coupled via an electromagnetic link to an implanted receiver-stimulator connected to one or more leads with one or more electrodes for stimulating tissue. The power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, is contained in the external controller—a hand-held sized device typically worn on the patient's belt or carried in a pocket. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation. In contrast, the fully implanted type of stimulating system contains the control circuitry, as well as a power supply, e.g., a battery, all within an implantable pulse generator (IPG), so that once programmed and turned on, the IPG can operate independently of external hardware. The IPG is turned on and off and programmed to generate the desired stimulation pulses from an external programming device using transcutaneous electromagnetic or RF links.
Whether an RF-controlled or fully implanted system is used, the electrode leads are implanted along the dura of the spinal cord. Individual wires within one or more electrode leads connect with each electrode on the lead. The electrode leads exit the spinal column and attach to one or more electrode lead extensions, when necessary. The electrode leads or extensions are typically tunneled along the torso of the patient to a subcutaneous pocket where the IPG or RF receiver-stimulator is implanted. The IPG or RF transmitter can then be operated to generate electrical pulses that are delivered, through the electrodes, to the dorsal column and dorsal root fibers within the spinal cord. The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array.
In order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. Once the leads are correctly positioned, a “fitting procedure” is performed by electronically programming the electrode array with a set of parameters that best addresses the painful site. Thus, electrode programming may be used to pinpoint the stimulation area correlating to the pain. Such electrode programming ability is particularly advantageous after implantation should the leads gradually or unexpectedly move, thereby relocating the paresthesia away from the pain site. With electrode programmability, the stimulation area can often be moved back to the effective pain site without having to reoperate on the patient in order to reposition the lead and its electrode array.
Electrodes can be selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). Other parameters that may be controlled or varied in SCS include the frequency of pulses provided through the electrode array, pulse width, and the amplitude of pulses delivered. Amplitude may be measured in milliamps, volts, etc., as appropriate, depending on whether the system provides stimulation from current sources or voltage sources. With some SCS systems, and in particular, SCS systems with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the pulse generator or receiver, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current (or voltage) in different relative percentages of positive and negative current (or voltage). Moreover, there may be some electrodes that remain inactive for certain electrode configurations, meaning that no current is applied through the inactive electrode. The number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of electrode configurations and electrical pulse parameters (together referred to as “sets of stimulation parameters”) to the user.
To facilitate such selection, the physician or clinician generally programs the IPG, external controller, and/or external patient programmer through a computerized programming station or programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware can have a transmitting coil attachment to actively control the characteristics of the electrical stimulation generated by the IPG or transmitter to allow the optimum stimulation parameters to be determined based on patient feedback and for subsequently programming the IPG, external controller and/or external patient programmer with the optimum stimulation parameters.
A known practice for current-controlled stimulation systems is to select a few basic electrode configurations by selecting the polarity (anode, cathode, or inactive) and fractionalized current (percentage of current) sourced or sunk by each electrode either manually or automatically, and then refining these basic electrode configurations by gradually adjusting the polarity and fractionalized current flowing through the electrodes.
For example, FIG. 1 illustrates four different electrode configurations (using 16 electrodes) that can be tested. Each row specifies the polarity on each electrode. In exemplary electrode configuration 1, electrode 3 is an anode, electrode 2 is a cathode, and the remaining electrodes are inactive, with electrode 3 sourcing 100% of the current and electrode 2 sinking 100% of the current. In exemplary electrode configuration 2, electrode 13 is an anode, electrode 5 is a cathode, and the remaining electrodes are inactive, with electrode 13 sourcing 100% of the current and electrode 5 sinking 100% of the current. In exemplary electrode configuration 3, electrodes 6 and 14 are anodes, electrodes 3 and 11 are cathodes, and the remaining electrodes are inactive, with electrodes 6 and 14 respectively sourcing 45% and 55% percent of the current, and electrodes 3 and 11 respectively sinking 30% and 70% of the current. In exemplary electrode configuration 4, electrodes 1, 6, 9, and 14 are anodes, electrodes 3, 4, 11, and 12 are cathodes, and the remaining electrodes are inactive, with electrodes 1, 6, 9, and 14 respectively sourcing 15%, 21%, 22%, and 42%, and electrodes 3, 4, 11, and 12 respectively sinking 15%, 21%, 22%, and 42% of the current. Each of the basic electrode configurations creates a stimulation region having a nominal position within the spinal cord tissue.
Each of the basic electrode configurations can be refined by gradually moving the current sourced or sunk by the anodes and cathodes to adjacent electrodes, thereby electronically steering the stimulation region relative to the nominal position to determine the optimum refined electrode configuration. Electrical steering can be performed in accordance with a steering or navigation table. For example, as shown in FIG. 2, a portion of an exemplary navigation table can be used to gradually modify the exemplary basic stimulation set 2 of FIG. 1. As can be seen, exemplary electrode configuration 2 is represented by stimulation set 291 in FIG. 2. Current can be incrementally moved from cathode electrode 5 to cathode electrode 6 as one steps downward through the navigation table, and from cathode electrode 5 to cathode electrode 4 as one steps upward through the navigation table. The step size of the current should be small enough so that steering of the current does not result in discomfort to the patient, but should be large enough to allow refinement of a basic electrode configuration in a reasonable amount of time. In the illustrated embodiment, the current step size is 5%.
While the navigation table illustrated in FIG. 2 dictates that the current will be shifted between electrodes in uniform current steps, the stimulation region will actually move in non-uniform steps (i.e., the distance moved between steps is not uniform). This non-uniformity may cause some of the portions of the navigation table to be largely redundant in terms of clinical effect, while other portions of the navigation table may move the stimulation region in relatively large steps, potentially skipping useful electrode configurations. This hypothesis gains support from a computational model and anecdotal reports from the field. The model suggests that shifting current between anodes only minimally moves the stimulation region (<5% of movement of the stimulation region), and that much of the stimulation region movement occurs when shifting current between adjacent cathodes. Reports from the field also suggest that most of the paresthesia changes occur during shifts in current between the cathodes. The model also indicates that movement of the stimulation region during cathode current shifting is not linear. For example, the model predicts that nearly half of the stimulation region movement occurs when current is shifted between 60/40 and 40/60 splits in current percentage between two cathodes. That is, half of the movement of the paresthesia change may occur in only four navigation steps assuming a current shifting resolution of 5%. Thus, some portions of the navigation table may have too high of a current shifting resolution, thereby unnecessarily increasing navigation time, while other portions of the navigation table may have too low of a current shifting resolution, thereby ignoring potentially relevant electrode configurations.
There, thus, remains a need for an improved method and system for shifting current between electrical stimulation electrodes.